// 背景图片
bg-img($url)
  background-image: url($url);
  background-size: cover;
  @media (-webkit-min-device-pixel-ratio: 2),(min-device-pixel-ratio: 2)
    background-image: url($url + "@2x.png");
    background-size: cover;

  @media (-webkit-min-device-pixel-ratio: 3),(min-device-pixel-ratio: 3)
    background-image: url($url + "@3x.png");
    background-size: cover;
